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TONE MODIFIER 8 Sheets-Sheet 8 Original Filed Oct. 31, 1955 UnitedStates Patent 0 3,178,502 TONE MODIFIER Melville Clark, Jr., 8 RichardRoad, Cochituate, Mass. Continuation of application Ser. No. 543,874,Oct. 31, 1955. This application Dec. 11, 1961, Ser. No. 160,968 21Claims. (Cl. 841.24)

This invention relates to tone modifiers for use with electrical musicalinstruments and the like, and in particular to apparatus for modifyingthe frequency spectrum of an audio-frequency electric signal having thefrequency components of a musical tone to produce vibrato, tremolo, andchoral effects.

This case is a continuation of application No. 543,874, filed October31, 1955 and now abandoned.

In many musical instruments, there is provided by various means anaudio-frequency electric signal having the frequency components of amusical tone, and this signal is supplied to one or more loud-speakers,which convert the electric signal into sound waves. Well-known examplesof such electrical musical instruments include electric organs, electricguitars, phonographs, tape recorders, and radios. In fact, any musicalinstrument can be made into an electrical musical instrument byproviding a microphone or other pickup, an amplifier to convert soundwaves into electric signals, and a loud-speaker system to convert theelectric signals back into sound waves. The present invention is usefulwith any of these musical instruments, and operates by modifying thefrequency spectrum of the audio-frequency electric signal to producecorresponding modifications of the musical tone produced by theloud-speaker system.

Various types of tone modification may be desirable at certain times toprovide a variety of musical effects. For example, modification of asolo tone may be desired for the production of a choral effect or thicktone, that is, the musical effect produced by a plurality of similarmusical instruments playing the same note simultaneously. Such tonemodification is especially useful in the case of electric organs thatare designed or adjusted to simulate the tone of some other musicalinstrument. As electric organs are commonly built, a given note of aparticular clavier can simulate but one instrument of a specified typeplaying that note. The musical capabilities of such an organ would beincreased if a given note played on it could simulate many musicalinstruments of one type playing the same note simultaneously. As anotherillustration, a small orchestra or band has only a limited number ofinstruments of each type. The musical capabilities of such an orchestraor band would be increased if it could produce the effect of many moreinstruments of each type playing substantially in unison. Accordingly,an object of this invention is to provide a tone modifier for modifyingsolo tones to provide choral effects.

Other musical effects are sometimes desired that may not be within thecapabilities of a particular musical instrument. For example, a musicianmay desire to produce a tremolo, that is, a cyclic amplitude variationof the musical tone. Or the musician may desire to produce a vibrato,that is, a cyclic pitch variation of the musical tone. Accordingly,another object of this invention is to provide improved tone-modifyingapparatus for producing tremolo and vibrato effects.

Another object of this invention is to provide tone modifiers forproducing timbre scintillations and other musical effects. As hereinused, the term timbre scintillation refers to a temporal variation ofthe timbre of a musical tone about its average character. Still furtherobjects and advantages of the invention will appear as the descriptionproceeds.

Briefly stated, and in accordance with one aspect of this invention,novel tone modifiers are provided that modulate an audio-frequencyelectric signal with a plurality of different subaudio frequencies toprovide various musical effects, including the thick tone or choraleffect of a plurality of similar instruments playing the same notesimultaneously. Subaudio frequencies suitable for this purpose generallylie within the range of 0.5 to 20 cycles per second. All frequencies upto 20 cycles per second are considered subaudio frequencies for purposesof this patent application, although subaudio frequencies in the rangeof 2 to 7 cycles per second are preferred and are generally employed. Inone embodiment of the in vention, hereinafter more fully described,modulation or modification of the tone is accomplished by means ofmagnetic recording apparatus having a plurality of oscillating heads bymeans of which the signal is pitch modulated with a plurality ofdifferent subaudio frequencies. In other embodiments hereinafterdescribed, modulation or modification of the tone is provided byelectrical networks having cyclically varying transmittancecharacteristics. In still another embodiment of the invention,modulation or modification of the tone is effected by the use of novelfrequency translation or carrier techniques. In general, the modulationof an audio-frequency tone or signal with subaudio frequencies providessidebands within narrow frequency spectra about each audiofrequencycomponent of the original or unmodified tone or signal.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings, and its scope willbe pointed out in the appended claims. In the drawings,

FIG. 1 is a plan view of novel magnetic recording apparatus that is partof a first embodiment of this invention;

FIG. 2 is a section taken along the line 2-2 of FIG. 1:

FIG. 3 is a section taken along the line 33 of FIG. 1;

FIG. 4 is a section taken along the line 4-4 of FIG. 1;

FIG. 5 is a circuit diagram of the first embodiment;

FIG. 6 is an elevation of a switch used in the first embodiment;

FIG. 7 is a schematic and circuit diagram of a second embodiment;

FIG. 8 is an elevation of a switch used in the second embodiment;

FIG. 9 is a schematic and circuit diagram of a third embodiment;

FIG. 10 is a block diagram of a fourth embodiment;

FIG. 11 is a schematic and block diagram of a fifth embodiment.

Referring now to FIGS. 1 through 4 of the drawing, a novel magneticrecording apparatus includes a magnetic drum 1 attached to a shaft 2that is continuously rotated at a constant speed by suitable means suchas an electric motor 3 connected to shaft 2 by a driving belt 4.Adjacent to the periphery of drum 1 there are a plurality ofconventional magnetic recording heads 5, 6, and 7, a conventionalreproducing head 3, and a conventional erasing head 9. Recording heads5, 6, and 7 are narrow heads spaced across the width of drum 1, as isbest shown in FIG. 1, to provide three parallel magnetic recordings onthe periphery of the magnetic drum. Reproducing head 8 is sufficientlyWide, at least in the portion thereof adjacent to drum 1, to receive allthree of the magnetically recorded signals, and provides a reproducedelectric signal having frequency components that are the sum of thefrequency components of the three magnetic recordings. Alternatively,three narrow side-by-side reproducing heads may be used and connectedtogether electrically to provide the same reproduced signal. Erasinghead 9 is sufficiently Wide to erase all three of the magneticrecordmgs.

In the embodiment illustrated, the reproducing head 3 and the erasinghead 9 are stationary, and these two heads are supported by stationarysupporting members 10 and 11 attached to the frame or housing (notshown) of the recording apparatus. Recording head 5 is attached to aU-shaped support 12 that is rotative on a pair of stationary sleeves 13and 14, held in fixed position by any suitable means (not shown), whichalso support the shaft 2. Consequently, recording head 5 is movable in acircumferential direction with respect to drum 1. In a similar way,recording head 6 is attached to a U-shaped support 15 that is rotativeon sleeves 13 and f4, and recording head 7 is attached to a U-shapedsupport 16 that is rotative on sleeves l3 and 14. Consequently, therecording heads 5, 6, and 7 are independently movable in acircumferential direction about drum 1.

A plurality of cams 17, 18, and 19 are attached to a shaft 20 that isrotated at constant speed by suitable means such as a driving belt 21connecting drums 2-2 and 23 carried by shafts 2 and 2%, respectively.Cam 17 operates a cam follower 24 pivoted on a stationary shaft 25 andconnected by a link 26 to the rotative support 12 that supportsrecording head 5. As cam 1'7 rotates, recording head 5 is oscillated ata subaudio frequency in the direction of motion of magnetic drum 1.

In a similar way, cam 18 operates a cam follower 27 pivoted on shaft 25and connected by link 28 to the rotative support 15 that supportsrecording head 6, so that as cam 18 rotates recording head 6 isoscillated at a different subaudio frequency in the direction of motionof magnetic drum 1. Cam 19 operates a cam follower 29 pivoted on shaft25 and connected by a link 39 to the rotative support 16 that supportsrecording head 7, so that as cam 19 rotates recording head i isoscillated at still another subaudio frequency in the direction ofmotion of drum 1.

Cam follower 29 is held in contact with cam 19 by the force supplied bya spring 31 connected between U- shaped member 16 and a stationarysupport 32, as is best shown in FIG. 2. Similar springs, not shown, areconnected between stationary support 32 and the U-shaped members 12 and15, so that cam followers 24 and 27 ac held in contact with cams 17 and18 respectively.

Drum It may be a conventional magnetic recording drum having on itsperipheral surface any suitable magnetic recording medium, such as acoating of a magnetic oxide. To provide a high-quality recording system,drum 1 preferably has a peripheral speed of about inches per second. Forexample, drum 1 may be about 10 inches in diameter and may be rotated ina clockwise direction, as indicated in the drawing by arrow 33: (FIG. 2)at a constant speed of 120 revolutions per minute.

Shaft 20 and each of the earns 17, 18 and 1% are rotated in a clockwisedirection at a constant speed of 12 revolutions per minute, or 1revolution each 5 seconds. The periphery of each cam has a contourcorresponding to a different plurality of substantially sinusoidalwaveforms, so that each of the recording heads 5, 6, and '7 isoscillated in a substantially sinusoidal manner at a different subaudiofrequency. For example, the contour of cam 19 includes 11 completesine-wave cycles, and rotation of the cams at 12 rpm. oscillatesreproducing head 7 at a subaudio frequency of 2.2 cycles per second. Thecontour of cam 18 includes 17 complete sine-wave cycles, as is shown inFIG. 3, and rotation of the cams oscillates recording head 6 at asubaudio frequency of 3.4 cycles per second. The contour of cam 17includes 25 complete sine-wave cycles, as is shown in FIG. 4, androtation of the cams oscillates recording head 5 at a subaudio frequencyof 5 cycles per second.

The oscillation amplitude of each recording head is determined by thewaveform amplitude of the associated cam and by the design or adjustmentof the cam-follower linkage. Preferably each recording head has anoscillai tion amplitude that is inversely proportional to itsoscillation frequency. For example, recording head 5 may be oscillatedthrough the total distance of about 40 mils along the periphery of drum1, while head 6 is oscillated through a distance of about 60 mils andhead 7 is oscillated through a distance of about mils.

As is hereinafter more fully explained in connection with PEG. 5, thesame audio-frequency signal may be supplied simultaneously to each ofthe recording heads 5, 6, and 7. Assume, for example, that a 4G0 cycleper second signal is supplied to recording head 7. The recording headproduces a magnetic signal on the surface of drum 1, which reproducinghead 8 converts into a reproduced electric signal. The wavelength of therecorded magnetic signal is proportional to the velocity of the drumperiphery relative to recording head '7, and is inversely proportionalto the frequency of the electric signal supplied to recording head 7.The frequency of the reproduced electric signal is inverselyproportional to the wavelength of the recorded magnetic signal.Accordingly, if recording head '7 and reproducing head 8 were bothstationary, while drum El rotated at constant speed, the frequency ofthe reproduced signal would be the same as the frequency of the signalsupplied to the recording head, and under the assumed conditions wouldbe 400 cycles per second.

However, as the recording head oscillates, the velocity of the drumrelative to the recording head varies at a subaudio rate, andconsequently the wavelength of the recorded magnetic signal also variesat a subaudio rate. Since the reproducing head 8 is stationary, thefrequency of the reproduced signal is modulated with a subaudiofrequency equal to the oscillation frequency of head 7, which in thecase under consideration is 2.2 cycles per second. The frequencydeviation of the reproduced signal is proportional to the oscillationamplitude and oscillation frequency of the recording head and, with thedesign parameters herein given, is approximately one percent of thecenter frequency. That is, when a 400 cycle per second signal issupplied to recording head 7, the reproduced signal provided byreproducing head i has a center or average frequency of 400 cycles persecond, while the instantaneous frequency of the reproduced signalvaries cyclically between limits of approximately 396 cycles per secondand 404 cycles per second. In other words, the reproduced signalcorresponds to a musical tone having an average pitch of 400 cycles persecond and having a vibrato with a frequency range of about 8 cycles persecond at a subaudio rate of about 2.2 cycles per second.

Now assume that an audio frequency of 800 cycles per second is suppliedto recording head 7. This audio frequency is likewise modulated with asubaudio frequency of 2.2 cycles per second, and its frequency deviationis about 1 percent of the center frequency, or 8 cycles per second toeach side of the center frequency making a total frequency range of 16cycles per second. When the audio-frequency signal supplied to recordinghead '7 has the frequency components of a complex musical tone, afundamental component of 460 cycles per second and an overtone or asecond harmonic component of 800 cycles per second, for example, thefrequency of each component is modulated with a sub-audio frequency of2.2 cycles per second, and the frequency deviation of each component issubstantially one percent of the center frequency of that component.When the instantaneous frequency of the fundamental component in thereproduced signal is 396 cycles per second, the instantaneous frequencyof the second harmonic component of the reproduced signal is 792 cyclesper second.

It is thus apparent that the original harmonic relation of an overtoneto the fundamental is maintained, and that the frequency deviation of anovertone is greater than that of the fundamental by the same ratio thatexists between the center frequency of the overtone and the centerfrequency of the fundamental. Consequently, a true pitch.

modulation is provided in which the pitch of the tone is cyclicallyvaried at a subaudio frequency wi hout changing the harmonic structureor timbre of the tone.

Thus pitch modulation is somewhat different from true frequencymodulation, since in frequency modulation the frequency deviation is nota function of the carrier frequency. If a signal having 400 cycle persecond and 800 cycle per second frequency components were frequencymodulated, the frequency deviation of both components would be the same,and in general the instantaneous frequencies would not have the sameharmonic relation as the original or center frequencies of the samecomponents.

In a similar manner, the audio-frequency signal supplied to recordinghead 6 is pitch modulated with a subaudio frequency of 3.4 cycles persecond, and the audio-frequency signal supplied to recording head 5 ispitch modulated with a subaudio frequency of 5 cycles per second. Thereproduced signal is a combination of these three pitch-modulatedsignals, and con equently represents a signal having a complexmodulation pattern corresponding to simultaneous pitch modulation of theoriginal signal with the three different subaudio frequencie In otherwords, the reproduced signal has a complex vibrato pattern such as wouldbe produced by playing the same note on three similar musicalinstruments simultaneously, with three different vibrato rates. Themusical eff ct of such complex vibrato patterns is a thick tone orchoral effect, so that a sound Wave produced from the reproducedelectric signal gives the musical impression of several similar musicalinstruments playing the same notes simultaneously.

Theoretically, an ideal situation for the production of pleasing choraleffects would be achieved by making the three subaudio modulatingfrequencies asynchronous, so there would be no periodic repetition ofthe modulation pattern. In practice, a satisfactory musical effect canbe obtained by making the period of the complex modulation patternrelatively long, 5 seconds for example. in the embodiment illustrated inFIGS. 1 through 4, this relatively long period of the complex modulationpattern is achieved by making the number of complete sinusoidalwaveforms in respective ones of the cam contours large integers havingno common factor, such as ll, 17, and 25, and by rotating the cams at arelatively slow speed, such as 12 rpm. With this arrangement there is norepetition in the modulation pattern during one compiete rotation of thecams, and the modulation pattern repeats itself only at the relativelyslow frequency of once each 5 seconds. Minor imperfections in the camcontours or in other parts of the modulating apparatus are generally notharmful, but merely add to the asynchronism or randomness of themodulation pattern. Conse quently, high precision cams are not requiredand manufacture of the magnetic recorder modulating apparatus can berelatively inexpensive.

When each recording head has an oscillation ampli ude inverselyproportional to its oscillation frequency, he frequency deviation of themodulated signal produced by each of the modulation frequencies issubstantially the same, preferably about 1 percent. Here also themanufacturing tolerances are not severe, since some variations in thedegree of modulation tend to enhance rather than to detract from thepleasing musical effect.

The erasing head 9 is supplied with a relatively highfrequency current,about 150 lzilocycles per second for example. This produces ahigh-frequency magnetic field across the gap of the erasing head thaterases the magnetically recorded si nals on drum 1, as is Well l: own bythose skilled in the magnetic recording art, and provides at all times asupply of unmagnetized magnetic medium to the recording heads.

Preferably, the recording heads 5, 6, and 7 are spaced apart along theperiphery of drum 1 at about 3 inch intervals. The signal recorded byrecording head 7 is reproduced by reproducing head 8 after a time delaydependin upon the rotational speed of drum 1 and the distance betweenhead 7 and head 8. The signal recorded by head 6 is reproduced after agreater time delay proportional to the distance between head 6 and head3, and the signal recorded by head 5 is reproduced after a still greatertime delay proportional to the distan e between head 5 and head 8.Consequently, when a signal representing a musical tone is supplied toheads 5, 6, and 7, simultaneously, a portion of this signal recorded byhead 7 is reproduced first, then after a time delay the signal recordedby head 6 is reproduced, and after a still further time delay the signalrecorded by head 5 is reproduced. This is musically desirable, sincewhen the same note is played upon three separate musical instruments,the three instruments do not begin to play the note at precisely thesame time, nor do they stop playing the note at precisely the same time.When the reproduced signal provided by head 8 is converted into soundwaves, a similar musical effect is obtained because of the differenttime delays involved in reproducing the signals recorded by the threeheads.

Various modifications in the magnetic recording and modulating apparatusare possible without departing from the inventive principles involved.For example, numerous variations can be made in the mechanicalstructure, such as the use of gear trains in place of belts tointerconnect the rotativc parts, the use of cranks in place of cams toproduce oscillatory motion, and the use of a magnetic tape, disc, or thelike, in place of the magnetic drum. Furthermore, instead of using aplurality of oscillatory recording heads with a single stationaryreproducing head, a single stationary recording head may be used with aplurality of oscillatory reproducing heads, or the recording andreproducing heads may both be oscillatory.

instead of three parallel heads each oscillated sinusoidally at adifierent subaudio frequency, a single recording system may be usedhaving only one recording head and one reproducing head. Mutliplemodulation of the signal may then be provided by oscillating either therecording head or the reproducing head, or both, in a complex mannercorresponding to the sum of several sinusoidal oscillations. Forexample, mechanical linkages may be provided to add all or selected onesof the motions of cam followers 2 27, and 29, and the combined motion soprovided may be used to oscillate a single recording head. Because ofthe time delay between the recording and reproducing operations,modulation of the signal may occur even if the recording head and thereproducing head are oscillated in synchronism with each other, and areseparated at all times by a constant distance. Alternatively, both therecording head and the reproducing head may be stationary, and the drumspeed may be modulated to pitch-modulate the reproduced signal.

in a broad sense, the magnetic recording drum is simply a delay ormemory device for transmitting and storing signals representing amusical tone, while the recording and reproducing heads are scanningdevices for scanning the signals transmitted by the delay device.Accordingly, other delay or memory devices, such as electrical delaylines, may be employed, in conjunction with scanning capacitors or thelike for periodically scannin the signals being transmitted by the delayline.

Instead of recording the audio-frequency signal directly, theaudio-frequency signal may first be amplitude-modulated upon acarrier-frequency signal, and the modulated carried may be recorded uponthe magnetic drum. In the reproduced signal, the carrier signal and itssidebands are pitch-modulated with the subaudio modulation frequenciesand upon subsequent demodulation of the carrier to recover theaudio-frequency components, it will be found that the audio-frequencysignal is pitch-modulated in the same manner as if the audio-frequencysignal had been directly recorded. With some magnetic recording 7 media,the use of such a carrier system improves the signal-to noise ratio.

Reference is now made to FIG. of the drawing, which illustrates theelectrical circuit of a tone modifier embodying principles of thisinvention. An electrical musical instrument 34 provides anaudio-frequency electri signal having the frequency components ot amusical tone. lnstrument 34- may be an electric organ, an electricguitar, or any other device providing an electric signal that may beconverted into sound waves for the production of musical tones. Theconversion of electric signals into sound waves is accomplished by oneor more lou speakers 35. The tone modifier herein described modifies themusical tone produced by loud-speaker by modifying the electric signalprovided by musical instrument 3d.

The complete tone modifier preferably includes a plurality of themagnetic recorders illustrated in FlG. l4-, connected in tandem in amanner to be described. For example, in the tone modifier illustrated,there are three of the magnetic recorders, which may all be similareXcept that their sub-audio modulation frequencies preferably aredifferent. For this purpose, one of the magnetic rccorders may havethree cams with contours of ll, 17, and 25 complete sine-wave cycles,respectively, while a second one of the three recorders has three camswith contours of 12, 19, and 29 complete sine-wave cycles, and the thirdone of the three recorders has three cams with contours of 13, 23, and31 complete sine-wave cycles. It should be noted that no pair of thesecam numbers have any common factor. Consequently, nine differentsubaudio modulation frequencies are provided, which for practicalpurposes are asynchronous.

The three magnetic recorders may be completely separate units except fortheir electrical interconnections, or they may have certain mechanicalparts in common. For example, all three recorders may use the same magnetic drum, with three sets of recording and reproducing heads atdifferent positions along its length.

.lternatively, the three magnetic recorders may have identical sets ofcamsthat is, the cam numbers may be the same in all three recorderslll,l7 and 25, for example. Asynchronism between the three sets ofmodulation frequencies can be achieved by rotating the three sets ofcams at slightly ditlerent speeds, by making the three driving pulleysfor the cams of slightly difierent diameters, for example, or byoperating the driving motors of the three recording systemsasynchronously.

The audio-frequency signal supplied by musical instrument 34 istransmitted by a lead as to four conventional vacuum-tube amplifiers 3?,38, 39, and 4 9. Amplifier 37 supplies this signal to a lead atconnected to the three recording heads 5, 6, and '7 of one magneticrecorder. Switches 4-2, 3, and 44 are connected between respective onesof these three recording heads and the ground lead 45', as shown, sothat the electrical circuit through each recording head is completedonly when the associated switch is closed. The reproduced electricsignal provided by reproducing head 8 is amplified by a conventionalvacuum-tube amplifier id and supplied to a lead 4'7 that also receivesthrough amplifier 3 5 the original signal provided by musical instrument3%. Consequently, lead 47 receives both the original signal and thereproduced signal provided by reproducing head 3.

The second magnetic recorder includes a continuously rotated magneticdrum 48, three oscillatory recording heads 49, 5t), and 51, areproducing head 52, and an erasing head 53. The signal provided at lead47 may be supplied to each of the recording heads 49, 5t and 51 througha plurality of switches 54, 55, and 56, connected between respectiveones of these recording heads and the ground lead :5, as shown. Thereproduced electric signal provided by reproducing head 52 is amplifiedby a conventional vacuum-tube amplifer 57 and is supplied to a lead 53that also receives the original signal from the musical instrument 34through amplifier 39.

The third magnetic recorder comprises a continuously rotated magneticdrum 59, a plurality of oscillatory recording heads so, til, and 62, areproducing head 63, and an erasing head 64. The signal provided at lead58 may be supplied to each of the recording heads 69, (,1, and 62through a plurality of switches 65, 66, and 67 connected betweenrespective ones of these recording heads and the ground lead 45, asshown. The reproduced electric signal provided by reproducing head 63 isamplified by .a conventional vacuum-tube amplifier 68 and is supplied toa lead that also receives the original signal from musical instrument El-l through amplifier 49. Lead as is connected to the loud-speaker 35 forconverting the electric signals into sound waves.

The three erasing heads 9, 53, and 64 are connected to erase oscillatorit that supplies a kilocycle per second erase signal to the threeerasing heads. Cons quently, each of the recording drums 1, 48, and 59is ,letely erased at each revolution. A normally closed switch "1 isconnected between amplifier 4t and lead 69,

- to a normally open switch '72 connected in ll or" the switches are inthe positions n in FIG. 5. S itches d2, 43, 44, 54, 55, 56, 65, 66, and'72 are open, w ile switch 71 is closed. The electric circuits throughall nine recording heads are now open,

duced electric signal that is transmitted through amplifier and lead toloudspeaker 35. This reproduced signal is pitchanodulated at a sub-audiofrequency, as hereinbefore explained, simulating a musical tone having avibrato. Loudspeaker 35 now receives two signals, unmodified signalthrough amplifier 4t and a modulated signal through amplifier 68, andthe musical tone produced by the loudspeaker corresponds to a mixture ofthese two signals. The instantaneous frequencies of the two gnals aredifferent, and as a result a musical choral fleet is produced similar tothat of two similar musical nstruments playing the same notesimultaneously.

Now assume that switches 66 and 67 are both closed.

reproduced signal supplied through lead 69 to lou 3-5 is nowpitch-modulated wih two diilerent submusical instruments playing thesame notes simultaneously with different vibrato rates. This signal ismixed with the unmodified signal supplied th ough amplifier to produce achoral effect similar to that of the three musical instruments playingsimultaneously. Similarly, when switches 65, 66, and 67 are all closed,a choral effect similar to at of four musical instruments playingsimultaneously is produced.

Now assume that switches 56, 65, 66, and 67 are all closed. Theunmodified audio-frequency signal provided by instrument 3% is suppliedthrough amplifier 38 to recording head 51, which records anaudio-frequency signal on magnetic drum 48. Reproducing head 52 nowsupplies a re: roduced signal through amplifier 57 to lead 58 that ispitch-modulated at a subaudio frequency. This modulated signal is mixedwith the unmodified signal supplied to lead 58 by amplifier 39, and thetwo signals are simultaneously supplied to each of the recording heads'51, and

As a result, the three signals recorded upon drum 59 are comple. eachrepresenting a combination of pitchmodulated and unmodulated signals.Oscillation of the heads 69, 61, and 62 further pitch-modulates thiscomplex signal, so that a signal is supplied to lead 69 throughamplifier 68 that has a relatively complex modulation pattern containingthe three modulation frequencies introduced by oscillation of heads 60,61, and 62 plus combinations of each of these three modulationtrenquencies with the modulation frequency introduced by oscillation ofrecording head 51. Consequently, six different modulaion frequencies arepresent in the signal transmitted by amplifier 68, and when this signalis mixed with the unmodified signal transmitted by amplifier 49, themusical tone produced by loud-speaker 35 has a choral effect similar tothat of seven similar musical instruments playing the same notessimultaneously. The vibrato pattern is now so complex that it isgenerally not recognizable as such, and the total effect produced is oneof thickness of tone.

Now assume that switches 55, 56, 66 and 67 are all closed. The signalssupplied to lead 58 represent th ee instruments playing simultaneously,and each of these signals is modulated by the three modulationfrequencies provided by oscillation of heads 69, 61, and 62, so thatamplifier 68 supplies to lead 69 a signal modulated with nine differentsubaudio frequencies. To this signal there is added the unmodifiedsignal supplied through amplifier 4f), so that loud-speaker 35 nowprovides a tone that simulates ten similar musical instruments playingthe same notes simultaneously.

As successively greater numbers of the switches are closed, the choraleffect produced is that of successively larger numbers of similarmusical instruments playing the same notes simultaneously. For example,closing switch 5d increases the number or" instruments represented inthe choral effect to 13, closing switch 44 increases the number ofinstruments represented to 22, closing switch 43 increases the number ofinstruments represented to 31, and closing switch 42 increases thenumber of instruments represented to 40. It will be noted that thetandem arrangement of the magnetic recorders multiplies the tonethickness produced by each, so that choral effects representing a verylarge number of musical instruments are produccd with relatively fewmodulators. By adding additional magnetic recording modulators in asimilar manner, choral effects representing any number of similarmusical instruments playing the same notes simultaneously can beobtained. The choral effect of thousands of instruments playingsimultaneously can be produced with relatively small, compact apparatusoperated by a single musician.

A still better understanding of the multiplicative effect of thetandem-connected modulators can be obtained by con dering theiroperation from the standpoint of modulation theory. The electricalmusical instrument 3% supplies an electric signal havin a plurality offrequency components corresponding to the frequency components of acomplex musical tone. Assume, for example, that there is a fundamentalcomponent having a frequency of 400 cycles per second and a secondharmonic component having a frequency of 800 cycles per second. Inpractice there will generally be many other frequency components, but aconsideration of two components is sufficient for illustrative purposes.Each of these frequency components is pitch-modulated with threedifferent subaudio frequencies by the oscillation of recording heads 5,6, and '7, so that the signal provided at lead 47 has, in addition tocomponents at the initial frequencies of 400 and 800 cycles per second,a plurality of sidebands symmetrically spaced in the frequency spectrumabout each of the initial frequency components and separated therefromand from each other by frequency differences equal to each of thesubaudio modulation frequencies plus multiples and combinations thereof.

In other words, modulation of the 400 cycle per second signal componentby oscillation of head '7 at a subaudio frequency of 2.2 cycles persecond produces sidebands spaced in frequency on each side of the 400cycle coni pouent at intervals of 2.2 cycles per second and multiplesthereof. Similarly, sidebands are produced on each side of the 800 cycleper second component. Oscillation of recording head 6 at 3.4 cycles persecond produces on each side of each frequency component sidebandsspaced at intervals or" 3.4 cycles per second, and oscillation of head 5at 5 cycles per second produces sidebands on each side of each frequencycomponent spaced at intervals of 5 cycles per second.

in addition, modulation with several frequencies simultaneously producessidebands spaced at intervals corresponding to combinations of themodulation frequencies, according to frequency-modulation theory. Thus,there is produced for each frequency component a complex spectrum offrequencies centered on the original frequency component, and having awidth substantially equal to twice the sum of the maximum frequencydeviation and the highest modulating frequency. Assuming a maximumfrequency deviation of 1 percent and a maximum modulation frequency of 5cycles per second, the frequency spectrum centered on the 400 cycle persecond component Will be about 18 cycles per second wide, while thatcentered on the 800 cycle per second will be about 26 cycles per secondwide.

The signal at lead 47 has a much larger number of frequency componentsthan the original signal provided by musical instrument 34, but sincethese components are grouped about the original frequency components,the pitch and timbre of the musical tone are not materially changed. Theprincipal effect produced by the tone modification is a thickening ofthe tone or a choral effect, similar to that produced by many similarmusical instru ments playing the same notes simultaneously.

Each of the frequency components present at lead 47 is modulated withthree different subaudio frequencies by oscillation of recording heads49, 5t and 51, to produce an even larger number of sidebands centeredabout each of the original frequency components. In a similar way, evenmore sidebands are added by the oscillation of heads 64., 61, and 62, sothat the signal supplied to loud-speaker has an exceedingly large numberof sidebands grouped about each of the original frequency components andspaced therefrom and from each other by different subaudio frequencydifferences. The result is an exceedingly thick tone, giving a choraleffect similar to that of a very large number of similar musicalinstruments playing the same notes simultaneously.

Now assume that switches 42, 43, 44, 54, 5'5, 56, 65, 65, and 67 are allopen, as shown in FIG. 5, but that switch 71 is open while switch 72 isclosed. The audio-frequency signal supplied by musical instrument 34 isnow pitchmodulatea at a single subaudio frequency by oscillation of head52, and this single pitch-modulated signal is supplied to loud-speaker35. Under these conditions the pitch of the musical tone produced variesperiodically at a subaudio frequency, 6.2 cycles per second for example,and the musical eifect is that of a single instrument played with a 6.2cycle per second vibrato. Consequently, the same tone modifier can beoperated to produce either choral or vibrato effects, or both,selectively, depending upon the switch positions. Switches 71 and 72 maybe operated by the musician to provide vibrato effects whenever sucheffects are desired.

Reference is now made to FIG. 6 which illustrates a convenient switchingarrangement for controlling the pro duction of choral effects. Switches42, 43, 44, 54, 55, 56, 65, 66, and 67 are formed from a stack of metalspring leaves arranged as shown and insulated from one another by stripsof insulation at one end of the stack. Leaf 42 is connected to recordinghead 5 by lead 5, leaf 43 is connected to recording head 6 by lead 6',and so forth, the lead to each recording head being identified by thesame reference numeral with a prime added. At the bottom of the stack,there is a metal spring leaf 45 which ears is connected to the groundlead 45. The stack of spring leaves is held in position by suitablemeans such as rivets 73 and 74 passing through the insulating stripsbetween the spring leaves into a stationary supporting member 75.

A choral pedal 76 is pivotally supported by a stationary shaft '77 andis biased to a rest position, as shown, by spring '78. When theleft-hand end of pedal 76 is depressed, the right-hand end of the :edalmoves spring leaf 45 upward into contact with spring leaf 67, therebyconnecting recording head 62 to the groun lead 45 and permitting anelectric signal to pass through recording head 62. Further depression ofpedal it; moves spring leaf 67 into contact with spring leaf 66,whereupon recording heads 61 and as are both connected to lead 45' andreceive electric signals from lead 53. Further depression of pedal 7econnects successive ones of the recording heads to lead 45, and therebyprovides successively greater thickness of the musical tone. in this waythe musician can easily control the thickness of the tone to providechoral efiects representing different numbers of similar musicalinstruments playing the same notes simultaneously.

Reference is now made to FIG. 7, which illustrates a somewhat differenttone modifier embodying principles of this invention. Electrical musicalinstrument 79 provides at lead fill an audio-frequency electric signalhaving the frequency components of a musical tone. This signal istransmitted through a resistor 31 to a lea $55, which is connected toground lead 83 by a resistor 34. The resulting voltage across resistoris amplified by a conventional vacuum-tube amplifier 85, which suppliesan electric signal to a l ad 855. T is signal is transmitted through aresistor 07 and a lead to a resistor S9, which is also connected toground lead The voltage across resistor 89 is amplified by aconventional vacuum-tube amplifier 99, which supplies an electric signalto one or more loud-speakers $1. Loud-speaker 9i converts the electricsignal into sound waves to produce a musical tone.

A variable transmittance electrical network comprises a rotary scanningswitch 92 having a plurality of commutator segments connected as shownto taps of a voltage-divider resistance network consisting of resistors93, 94, $5, 96, 9'7, 58, 99, and 1%, connected as shown between lead andlead 83. The contact arm of scanning switch 92 is continuously rotatedat low speed, 2.2 revolutions per second, for example, by a train ofgears 1M and 1% connected to a shaft 1% that is continuously rotated atconstant speed by suitable means such as an electric motor llld.Consequently, the electrical transmittance of the network is modulatedor varied at a subaudio frequency of 2.2 cycles per second. The contactarm of scanning switch 92 is connected to lead 82 through a resistor 105and a normally open switch res. When switch 1&5 is closed, scanningswitch @2 supplies across resistor 84 a signal that is amplitudemodulated with the subaudio frequency of 2.2 cycles per second, whichsignal is in addition to the unmodulated signal transmitted throughresistor 31. Consequently, whenever switch res is closed, two electricsignals are transmitted simultaneously to loud-speaker 3'31, one ofwhich is amplitude modulated at a subaudio frequency while the other isunmodulated. if desired, the parallel volt age dividers 93% and FF-lu lmay be combined into one voltage divider to reduce the number ofresistors by a factor of two.

A similar variable transmittance network includes a' second scanningswitch M37 that is similar to switch $2 except that it is rotated at adifierent low speed, 3.4 cycles per second for example, by a train ofgears @3465: connected to shaft 193. Consequently, the transmittance ofthis second network is modulated at a subaudio frequency of 3.4 cyclesper second. Switch 107 is connected to lead by a resistor 11s: and anormally open switch ill. When switches 1'36 and 111 are both closed,

CHI

l2 three signals a e transmitted simultaneously to loudspeaker 9, afirst one of wh ch is amplitude modulated with a subaudio frequency of2.2 cycles per second, a second one of which is amplitude modulated at asubaudio frequency of 3.4 cycles per second, and the third one of whichis unmodulated.

Another similar variable transmittance network includes a third selectorswitch 112 that is similar to selector switch 92 except that it isrotated at still another low speed, 5 cycles per second, for exam le, bya train of gears 11., 14 connected to shaft 363. The contact arm ofswitch 112 is connected to lead through a resistor 125.5 and a normallyopen switch Elle. Fiiien switch 116 is closed, there is transmitted toloud-speaker 91 an additional signal which is amplitude modulated with asubauuio frequency of 5 cycles per second. If desired, the voltagedividers connected to the commutator segments of scanning switches 92,197 and 112 may be combined into a single voltage-divider network forreducing the number of resistors.

Other similar variable-transmittance networks include scanning switches12?, H3, and 113 which are rotated at still other low speedscorresponding to different su audio frequencies by gear trains connectedto shaft 1%, as shown. These scanning switches are connected to leadthrough resistors 11%, 12.1 and 322, in serie with noramlly openswitches 123, 124 and 12-5, respectively. The resistance networksassociated with scanning svitches 1 3, and 319 are connected betweenlead as and lead 33, so that these scanning switches amplitude modulateall of the electric signals or signal components supplied to lead Thisprovides a multiplication of the modulation frequencies, in the n annerhereinoe ore described in connection with 5, so that a very thick toneis produced by loudspeaker 91 when all of the switches rss, 111, li 123,124 one 125 are closed. Additional scanning switches may be provided tofurther multiply the thickness of the tone to desired extent.

Assume that the audio-frequency signal supplied to lead frequencycomponents of 400 cycles per second and 809 cycles per second. Thesefrequency components are transmitted through amplifiers 35 and 9% toloud-speaker $3, so that when all of the switches are open the toneproduced by the loud-speaker has the same frequency compone ts as theelectric signal supplied by the elecrical musical instrument. Whenswitch ass is closed, each frequency com onent of the original signal isamplitude modulated by the rotation of scanning switch thereby producingfor each of the original frequency components two side bands spaced oneither side thereof in the frequency spectrum at a frequency diferenceof 2.2 cycles per second. Such sidebands represent simple amplitudemodulation, and the tone produced under such circumstances byloud-speaker 91 is like that of a single musical instrument played withthe tremolo of 2.2 cycles per second.

When switches res and Ill are both closed, there are produced twosidebands on each side of each frequency component, spaced therefrom atfrequency differences of 2.2 and 3.4 cycles per second, respectively.Under such circumstances the tone produced by loud-speaker 9i isamplitude modulated with two subaudio frequencies simultaneously, andthe musical erlect is produced of two similar instruments playing thesame notes simultaneously with different tremolo rates. Similarly, whenswitches 1%, 111, and are all closed, the tone is modulated with threedifierent subaudio frequencies, and a choral effect representing threesimilar musical instruments playing the same notes simultaneously isprovided.

When all three of the switches rss, 111, and 116 are closed, the signaltransmitted by amplifier 85 to lead as consists of the originalfrequency components provided by musical instrument 7?, plus sixsidebands for each of the original frequency components, so that thesignal at lead has seven times as many frequency components as thesignal at lead 89. When switch 123 is closed, all of the frequencycomponents present at lead 86 are amplitude modulated with a subaudiofrequency by the network including scanning switch 117, thereby providing a pair of sidebands for each frequency component supplied to lead86. Now there are 21 frequency components for each of the originalfrequency components supplied to lead 80. When switch 124 is closed,another pair of sidebands is provided for each of the frequencycomponents provided at lead 86, and when switch 125 is closed stillanother pair of sidebands is provided for each of the frequencycomponents at lead 86. The multiplication of frequencies thus producedprovides a choral effect that represents a large number of similarmusical instruments playing the same notes simultaneously.

When all of the switches 1'56, 111, 116, 12 3, 124, and 125 are closed,there is provided for each of the original frequency components a totalof 48 sidebands spaced at subaudio frequency intervals within a smallfrequency spectrum centered on the original frequency component. Thetotal width of each such spectrum is equal to twice the sum of thehighest modulation frequency provided by scanning switches 92, 1117, and112 and the highest modulation frequency provided by the scanningswitches 117, 118, and 119. If the modulation frequency provided. byswitch 112 is five cycles per second, and is higher than that providedby switches 92 and 167, and the scanning frequency provided by switch119 is 6.2 cycles per second, and is higher than that provided byswitches 117 and 118, the total width of the frequency spectrum ofsidebands about each modulation component is 22.4 cycles per second.

FIG. 8 illustrates a choral effects control that may be used in theapparatus shown at FIG. 7. Switches 1%, 111, 116, 123, 124 and 125consist of a stack of metal spring leaves arranged as shown andinsulated from one another by strips of insulation. An additional metalspring leaf S2 is connected to lead 82, and still another metal leaf 88'is connected to lead 88 as shown. A choral pedal 126 is pivotallysupported by a stationary shaft 127. When the eft-hand end of pedal 126is depressed, the right-hand end of the pedal presses upward against aninsulating strip 12% attached to the bottom of leaf 82 and forces leaf82' into contact with leaf 166, thereby connecting resistor 165 to lead8-2. Further depression of the choral pedal moves leaf 195 into contactwith lead 111, and thereby connects resistor 110 to lead 82. Stillfurther depression of the pedal moves lead 111 into contact with leaf116 and con nects resistor 115 to lead 82. An insulating strip 129insulates leaf 116 from leaf 88' at all times. Accordingly, stillfurther depression of the choral pedal moves leaf 88' into contact withleaf 123 and connects resistor 120 to lead 88. Still further depressionsof the choral pedal connect resistors 121 and 122 to lead 88.

Reference is now made to FIG. 9 of the drawing, which illustrates stillanother tone modifier embodying principles of this invention. Electricalmusical instrument 135) supplies to lead 131 an audio-frequency electricsignal having the frequency components of a musical tone. This signal istransmitted through resistor 132 and lead 133 to a resitor 134, so thata voltage appears across resistor 134 that has all of the frequencycomponents present in the signal supplied to lead 131. This signal isamplified by an amplifier 135 that supplies an electric signal throughlead 135, resistor 137 and lead 138 to a resistor 139 across which avoltage appears that has all of the original frequency components. hissignal is amplified by a conventional vacuum-tube amplifier- 14i) and issupplied to one or more loudspeakers 141, which produce sound wavescorresponding to the electric signal.

The signal at lead 131 is supplied to a plurality of variabletransmittance formant circuits having transmittances that are a functionof frequency. One such formant circuit comprises resistors 14 2 and14-3, a switch 144 a capacitor 145, and a variable inductor 146,connected as shown. Capacitor 145 and inductor form a tuned circuit thatis parallel-resonant at an audio frequency. Consequently, the parallelimpedance of capacitor 145 and inductor 146 is different for differentaudio frequencies, and the signal supplied to resistor 143 is a modifiedsignal having fre quency components with relative amplitudes differentfrom the relative amplitudes of the corresponding frequency componentsin the original signal supplied to lead 131. It will be appreciated thatformant circuits of other types may also be used, including formantcircuits that do not include resonant selections but nevertheless havetransmittances that are functions of frequency.

The inductance of inductor 146, and therefore the resonant frequency ofthe tuned circuit, is cyclically varied at a subaudio frequency byadjustment means connected through a train of gears 147448 to a shaft149 that is con tinuously rotated at constant speed by suitable means,such as an electric motor 150. F or example, inductor 146 may comprise acoil on a magnetic core havin an air gap within which a non-circular oreccentric magnetic rotor is continuously rotated for cylically varyingthe reluctance of the magnetic circuit. As a result, this formantcircuit modulates the amplitude and phase of each frequency component inthe original audio-frequency signal with a subaudio frequency, whichmaybe 2.2 cycles per second, for example. Components near the resonantfrequency may be amplitude modulated at twice this frequency. Theamplitude and phase of this. modulation is different for differentfrequency components in the original signal, where by the timbre of thetone is modulated or scintillated. Consequently, the signal provided atresistor 143 has a varying or scintillating timbre. Alternatively,instead of varying the inductance of inductor 146, the capacitance ofcapacitor 145 or the resistance of resistor 142 may be varied cyclicallyat a subaudio frequency.

When considered in terms of modulation theory, the effect of formantcircuit 142-146 is to produce sidebands about each frequency componentof the original signal separated therefrom and from each other bysubaudio frequencydifferences. The sidebands of each frequency componentdiffer in their relative amplitude and phase relations from thesidebands of other frequency components, thereby providing a modulationor scintillation of timbre. When switch 14 is closed, the modulatedsignal is combined with the original unmodulated signal to produce achoral effect.

A second formant circuit comprises resistors 151 and 152, a normallyopen switch 153, a capacitor 154, and a variable inductor connected asshown. Capacitor 154 and inductor 155 constitutes a second tuned circuitthat is parallel resonant at an audio frequency different from theresonant frequency of the circuit comprising capacitor 145 and inductor145. The inductance of inductor 155, and therefore the resonantfrequency of circuit 154-155, is cyclically varied by gear train15-34157, connected to shaft 149, at a subaudio frequency different fromthe modulation frequency of illdtlCLOI' 146. F or example, theinductance of inductor 155 may be modulated with a subaudio frequency of3.4 cycles per second. Thus, when switch 153 is closed there is suppliedto lead 133 still another signal having a timbre that is modulated orscintillated at a nifferent frequency. Accordingly, additional sidebandsare added to the signal provided at lead 133, to provide a choral effectwith a somewhat thicker tone than was pro vided when only the switch 144was closed.

Still another formant circuit comprises capacitor 158 and a variableinductor 159 tuned to still another audio frequency, with the inductanceof inductor 159 modulated with still another subaudio frequency.Consequently, still another group of sidebands is added about eachfrequency component of the original signal when switch 161 is closed.When switches 144, 153, and 169 are all closed, a signal is provided atlead 136 that includes the original frequency components plus aconsiderable number of sidebands.

A further variable formant circuit comprises a capacitor 161 and avariable inductor 162 having an inductance that is modulated with stillanother subaudio frequency. Consequently, when switch 163 is closed,additional sidebands are provided for each frequency component at lead136, which includes the frequency components of the original signal plusthe sidebands provided by the first three variable formant circuits.Still another variable formant circuit comprises a capacitor 164, avariable inductor res having its inductance modulated with still othersidebands about each frequency component supplied to lead rse. A sixthvariable formant circuit comprises a capacitor 167, a variable inductorres ha ing an inductance modulated with a sixth subaudio frequency, anda switch 169 that may be closed to provide even more sidebands abouteach frequency component. Any numer of additional variable formantcircuits may be added as desired, to provide any desired number ofsidebands about each of the original frequency components.

When all of the switches M4, 153, 16%, 53, 166, and 169 are closed, thesignal supplied to loud-speaker 141 has a very large number of sidebandsabout each of the original frequency components, so that the choraleffect of a very thicc tone is produced similar to that of a largenumber of similar musical instruments playing the same notessimultaneously. Switches 144, s53, 16%, 163, 166, and 169 may beoperated by a choral pedal similar to that shown in PEG. 8.

The embodiments hereinbefore described illustrate the production ofchoral effects, vibrates, tremolos and timbre scintillations by directmodulation of an audio-frequency signal. Choral effects and othermusical effects can also be obtained by indirect modulation of theaudio-frequency signal effected by carrier-current techniques, as isillustrated in two embodiments that will now be described.

in FlG. 10 of the drawing, an electrical musical instrument 357iprovides at lead 1?]. an audio-frequency electric signal having thefrequency components of a musical tone. An oscillator 172 supplies tolead 173 an el ctric signal having a carrier frequency, 100 kilocyclesper second, for example. For purposes of this patent application, theterm carrier frequency" refers to any frequency that is higher than thehighest audio-frequency component of the musical tones that areproduced. This carrier-frequency signal is transmitted by a bufferampliher 174, a lead 1'75 and a normally closed contact of atwo-position switch 176 to a balanced modulator 177. The audio-frequencysignal at lead l'il is also sup, d to balanced modulator 177, whichheterodynes the carrierfrequency and audio-frequency signals to provideat lead 178 a plurality of sidebands having frequencies equal to thesums and differences of the carrier frequency and each frequencycomponent of the audio-frequency signal.

These sidebands are transmitted by lead 1 8 to a mixer 179 thatalsoreceives the carrier-frequency signal from lead 173 through a bufferamplifier Mixer 1'79 eterodynes the sideband frequencies with thecarrier frequency and supplies to an audio amplifier 181 an electricsignal having the same frequency components as those in the originalaudio-frequency signal supplied to lead 171 by musical instrument 17%.Consequently, the apparatus thus far described is similar to aconventional car- 'rier-current communications system.

A similar carrier-current system comprises a balanced modulator 182 thatreceives an audio-frequency signal from amplifier 181 through lead 1%and receives the carrier-frequency signal from lead 173 through a bufferamplifier 134 and a lead 185. Sidebands corresponding to the sum anddifference frequencies are provided at lead 186, and are supplied to amixer 187 that also receives the carrier-frequency signal from lead 173through a buffer amplifier 183. Mixer 187 supplies an audio-frequencysignal trrough lead 18% to an audio amplifier 1% and a loud-speakersystem 191 that produces sound waves corresponding to theaudio-frequency electric signal.

Assume, for example, that musical instrument 17% provides an electricsignal corresponding to a simple musical tone having a frequency of 490cycles per second. Oscillator 172 supplies a carrier-frequency signal at100 kilocycles per second. Modulator 177 heterodynes the audiofrequencyand the carrier-frequency signals, and supplies through lead 178 twosidebands having frequencies of 99,668 and 190,400 cycles per second,respectively. Mixer l7? heterodynes the two sidebands with the carrierfrequency of 109 he, vand provides a 460 cycle per second audiofrequencysignal that is transmitted by audio ampli'lier and lead 133 to balancedmodulator r32.

Modulator 182 heterodynes the 46-0 cycle per second audio-frequencysignal with the 106 kc. carrier frequency signal, and supplies to lead18% two sidebands having irequencies of 99,600 and 100,400 cycles persecond, respectively. dixer i237 heterodynes these sidebands with the100 lac. carrier-frequency signal, and supplies through lead 185* andaudio-amplifier 19% a 400 cycle per second electric signal that isconverted by loudspeaker 191 into a musical tone having a frequency of466 cycles per second. Consequently, in this mode of operation of thePEG. l0 pparatus, the signal provided by musical instrument 17% 'stransmitted to loud-speaker 191 without any modificaicn of tone.

An oscill tor res supplies to lead 192 a second carrierfrequency si nalthat differs from the carrienfrequency supplied to lead 173 by asubaudio frequency. For example, if oscillator 172 provides a signalhaving a frequency of 100 he, oscillator 13 2. may provide a signalhaving a frequency of 100,603 cycl s per second. This secondcarrier-frequency signal is transmitted through a buffer amplifier 193to a normally open contact of switch 176.

Now assume that switch 17o is operated to open its normally closedcontact while simultaneously closing its normally open contact. Now thecarrier-frequency signal supplied to modulator 177 has a frequency of160,003 cycles per second, and this signal is heterodyned with the 400cycle per second audio-frequency signal supplied by musical instrument17b to produce through lead lid a pair of sidebands having frequenciesof 99,693 and 100,493 cycles per second, respectively. These twosidebands are heterodyned with the 100 lrc. carrier-frequency signal bymixer 179, whereupon mixer 11 9 supplies to audio amplifier 181 a signalhaving audio-frequency components representing the differencefrequencies between the two sidebands and the 106' kc. carrierfrequency, In other words, the audio-frequency signal supplied toamplifier now has two frequency components, one of which has a frequencyof 397 cycles per second while the other has a frequency of 403 cyclesper second.

These two audio-frequency components are transmitted by the secondcarrier system to loud-speaker 191, which thereupon produces a complexmusical tone having a 397 cycle per second component and a 463 cycle persec ond component. The result is a choral effect, similar to thatproduced by two similar musical instruments playing the same notesimultaneously with slightly different frequencies. In other words, thecarrier system has modulated the original audio-frequency signal toprovide two sidebands each differing from the original audio-frequencysignal by a subaudio frequency difference of 3 cycles per second. In thicase the original signal is suppressed, which corresponds tosuppressed-carrier amplitude modulation. If the audio-frequency signalsupplied by musical instrument 17% has a plurality of frequencycomponents corresponding to a complex musical tone, a pair of sidebandsis produced in a similar manner for each of the ori inal frequencycomponents.

A switch 194 is connected in parallel with the normally closed contactof switch 176, as shown. Assume that the normally open contact of switch1% is closed and that switch 194 is also closed. Now twocarrier-frequency signals are supplied simultaneously to balancedmodulator 177, one of which has a frequency of 100,000 cycles per secondwhile the other has a frequency of 100,003 cycles per second. Whenmusical instrument 170 supplies a 400 cycle per second audio-frequencysignal to balanced modulator 177 through lead 171, four sidebands areprovided at lead 178, having frequencies of 99,600; 99,603; 100,- 400;and 100,403 cycles per second, respectively.

Mixer 179 heterodynes these four sidebands with the 100 kc. car-rierfrequency signal and supplies to audio amplifier 181 an audio-frequencysignal having components equal to each of the three audio-frequencydifferences; namely 397, 400 and 403 cycles per second. These threeaudio-frequency components are transmitted by the second carrier systemto loud-speaker 191, which thereupon produces a musical tone having thethree frequency components. It will be noted that these three frequencycomponents consist of the original frequency, 400 cycles per second, anda sideband on either side thereof in the frequency spectrum and spacedtherefrom by a subaudio frequency difference of 3 cycles per second.This corresponds to a simple amplitude-modulated signal, and pro vides atremolo effect.

Still another oscillator 195' supplies a third carrier-frequency signalthat differs from the first and second carrierfrequency signals bysubaudio frequency differences. For example, oscillator 195 may providea signal having a frequency of 99,995 cycles per second. This oscillatori connected to balanced modulator 177 through a buffer amplifier 196 anda normally open switch 197. Now assume that switch 194 is open and thatthe normally open contacts of switches .176 and 197 are closed. Alsoassume that the musical instrument 170 supplies a 400 cycle per secondaudio-frequency signal to balanced modulator 177. Modulator 177heterodynes the audio-frequency signal with the two carrier-frequencysignals that are supplied to it under these conditions, and provides atlead 178 four sidebands having frequencies of 99,595; 99,603; 100,395and 100,403 cycles per second, respectively. Mixer 179 heterodynes thesefour sidebands with the 100 kc. carrierfrequency signal and supplies toaudio amplifier 181 an audio frequency signal having frequencycomponents of 395, 397, 403, and 405 cycles per second. These four audiofrequency components are transmitted to loudspeaker 19 1, whichthereupon produces a musical tone having four frequency componentsseparated by a plurality of subaudio frequency differences. The musicaleffect is that of a thick tone or choral effect.

If switch 194 is also closed, the 100 kc. carrier-frequency is suppliedto modulator 177 along with the two carrier frequencies supplied byoscillators 192 and 195.

In this case the same four audio-frequency sidebands are transmitted toloudspeaker 191, and in addition the original 400 cycle per second audiofrequency signal is supplied to the loudspeaker. The resulting signal,comprising the original signal and two pairs of sidebands, is identicalto a signal produced by amplitude modulation of the original signal withtwo different subaudio frequencies simultaneously, so that the musicaltone produced by loudspeaker 191 contains a double tremolo similar tothat produced by two similar musical instruments playing the same notessimultaneously with different tremolo rates.

Still other oscillators 1'98 and 199 provide two additional.carrienfrequency signals that differ from all the othercarrier-frequency signals by subaudio frequency differences. Oscillator198 is connected to lead 185 through a buffer amplifier 200 and anormally open switch 201. Oscillator 199 is connected to lead 185through a buffer amplifier 202 and a normally open switch 203. Whenswitch 201 is closed, two different carrier frequencies aresimultaneously supplied to the balanced modulator 182,

whereupon the second carrier system provides two sidebands for eachfrequency component in the audio-frequency signal supplied to modulator182. through lead 183. When switch 203 is closed a thirdcarrier-frequency signal is supplied to balanced modulator 182,whereupon another is if? pair of sidebands is provided for eachfrequency component of the audio-frequency signal supplied through lead183. Consequently, when all of the switches are closed a large number ofsidebands are provided for each frequency component of the originalaudiofrequency signal supplied by the electrical musical instrument 170,and a very thick tone is produced by loud-speaker 191 to p oduce achoral effect similar to many musical instruments of the same typeplaying the same notes simultaneously.

The side-bands of each frequency component occupy a small frequencyspectrum centered on the original frequency of that component and havinga width equal to twice the sum of the larger of the two subaudiofrequency differences between the frequency of oscillator 172 and thefrequency of oscillators 192 and 195 and the larger of the two subaudiofrequency differences between the frequency of oscillator 17?. and thefrequencies of oscillators 198 and 199.

Oscillators 1'72, 192, 195, 198, and 199 may be oscillators of any typeproviding reasonably stable electric signals of appropriate frequency.The five oscillators are tuned to substantially the same frequency, butoperate asynchronously and therefore inevitably operate with slightfrequency differences. If these inherent frequency differences are notas large as desired, the oscillators may be slightly de-tuned withrespect to one another. The principal purpose of the buffer amplifiersis to isolate each oscillator from the others so that interactionbetween the oscillators will not inadvertently lock them intosynchronism. If electron-coupled oscillators or the like are ernployedsome or all of the buffer amplifiers may be omitted. Alternatively,passive networks may be used in place of buffer amplifiers to achievesignal isolation.

The amplifiers, modulators, and mixer-s may be of conventional typeswell known in the communications art. The mixers may be any devices thatproduce a heterodyning action or the equivalent between two sets ofinput signals, and may comprise multigrid mixer tubes of the typecommonly used in communications work, or they may be demodulators ordetractors of any suitable type, many of which are very well known.Since many suitable types of oscillators, amplifiers, modulators andmixers are well known, any further description thereof would besuperfluous.

Various modifications of the circuit shown in FIG. 10 are possiblewithout departing from the invention in its broader aspects. Forexample, a single carrier-frequency may be supplied at all times to themodulators, while a plurality of carrier-frequency signals are suppliedto the mixers for producing audio-frequency sidebands. If desired, aplurality of carrier-frequency signals can be supplied to the modulatorsand the same or a different plurality of carrier-frequency signals maybe supplied to the mixers, provided reasonable precautions that will beapparent to competent engineers are observed to prevent the suppressionof some difference frequencies by others. If unbalanced modulators areused instead of balanced modulators, the carrier-frequency signals willnot be suppressed, and accordingly the carrier frequencies as well asthe sidebands will be present in leads 178 and 186. Con sequently,subaudio difference frequencies will also be produced by the mixers, butthese are not necessarily objectionable since they can easily beeliminated, if so desired, by appropriate design of the audioamplifiers. In any event, subaudio components will seldom be reproducedto any considerable extent by conventional loud-speakers, and even ifthey are reproduced, they are inaudible.

Reference is now made to FIG. 11 of the drawing, which illustrates anembodiment wherein multiple carrier frequencies are produced bymodulating a single carrierfrequency signal. Electrical musicalinstrument 204 supplies to lead 205 an audio-frequency signal having thefrequency components of a musical tone. An oscillator 206 supplies tolead 207 an electric signal having a carrier frequency, kilocycles persecond, for example. The

215 or its circuit equivalent.

, that convert the electric signal into sound waves. apparatus thus fardescribed is a single-sideband carrier audio-frequency andcarrier-frequency signals are supplied to a balanced modulator 208 thatprovides at lead 20? a .pair of carrier-frequency sidebands for eachfrequency component of the audio-frequency signal. For example, if theaudio-frequency signal has a component at 400 cycles per second, a pairof sidebands are provided through lead 209 having frequencies of 99,600and 100,400 cycles per second, respectively.

A single-sideband filter 210 attenuates one sideband of each pairrelative to the other sideband, so that there is provided at lead 211one carrier-frequency sideband for each frequency component of theaudio-frequency signal.

The sideband at 211 may be either the sum or the difference frequency ofthe audio-frequency and carrierfrequency signals. Assume, for example,that the signal .at lead 211 has a sideband at 100,400 cycles per secondfor a 400 cycle per second audio-frequency component, a sideband at100,800 cycles per second for an 800 cycle per second audio-frequencycomponent, and so forth. This part of the apparatus is similar to thetransmitting portion of a conventional single-sideband communicationssystem, and any further description thereof would b superfluous.

The same carrier-frequency signal is also supplied to a center-tappedinductor 212. The center tap of inductor 212 is connectedto a lead 213,while its end terminals are respectively connected to a lead 214 and toa ground lead Inductor 212 supplies carrier-frequency signals to leads213 and 214, the carrier frequency voltage between leads 214 and 215being substantially twice as large as the carrier-frequency voltagebetween leads 213 and 215.

A portion of the carrier-frequency signal between leads 213 and 215 istransmitted through a resistor 21 5, a normally closed contact of atwo-position switch 217, and a lead 218 to the primary 219 of atransformer having a center tapped secondary 220. Primary 219 isconnected between leads 218 and 213. The center tap of secondary 220 isconnected to a lead 221, while its end terminals are respectivelyconnected to a lead 222 and to lead 215, as

. shown. Secondary 220 supplies carrier-frequency signals to leads 221and 222, the carrier-frequency voltage between leads 222 and 215 beingsubstantially twice as large as the carrier-frequency voltage betweenleads 221 and 215.

A portion of the carrier-frequency signal between leads 221 and 215 istransmitted through a resistor 223 and a lead 224 to a transformerprimary 225 connected between lead 224 and lead 221, as shown. Atransformer secondary 226 that is inductively coupled to primary 225.transmits this signal through a conventional vacuum tube amplifier 227to a mixer 228 that heterodynes the carrierfrequency signal'with thesidebands transmitted through lead 211. The difference frequencies aresupplied through an audio amplifier 229 to one or more loud-speakers 230The system that transmits audio-frequency signals from electricalmusical instrument 204 to loudspeaker 230 without substantialmodification, so that loud-speaker 2312 produces musical tonescorresponding to the electric signals supplied to lead 205.

Connected in series between lead 214 and ground lead 215 there is avariable phase-shifting circuit comprising a resistor 231, an inductor232, and a variable capacitor 233. A normally open switch 234 and aresistor 235 are connected in series between lead218 and the circuitjunction 23:? of resistor 231 and inductor 232. Switch 234 is alsoconnected to a normally open contact of switch 217, as shown.

Capacitor 233 may be a standard, commercially available type of variablecapacitor having one or more stationary plates and one or more movableplates that may be continuously rotated to vary its capacitancecyclically. The movable plates of capacitor 233 are continuouslyrosupplied through amplifier 229 to loud-speaker 230.

tated at a constant speed, 2 revolutions per second for example, to varythe capacitance cyclically at a subaudio frequency. Suitable means forrotating the movable capacitor plates may comprise a drum 237 rotated atconstant speed by any suitable means, such as a driving belt 233 drivenby an electric motor 239 and a driving pulley 240. To keep belt 233 tautand in good frictional engagement with drum 237, there is provided anidler pulley 241 rotatively mounted on an arm 242 pivotally supported bya stationary shaft 243. Ann 242 and pulley 241 are urged downward by aspring 244 to keep belt 238 under constant tension.

At or near its mid-capacitance value, capacitor 233 preferably isseries-resonant with inductor 232 at the frequency of the carrier signalprovided by oscillator 206. As the movable plates of capacitor 233 arerotated to vary its capacitance at a subaudio frequency, the impedancevof resonant circuit 232-233 is varied or modulated in amplitude andphase at subaudio frequencies. Consequently, the voltage across theresonant circuit is likewise modulated in amplitude and phase. Thevoltage between circuit junction 236 and lead 213 is modulated in phasewith a subaudio frequency, but has relatively little amplitudemodulation. For present purposes, with one exception hereinafterdiscussed, there is no material difference between phase modulation andfrequency modula- Accordingly, the circuit including capacitor 233 rierfrequency with a subaudio frequency.

Now assume that the normally closed contact of switch 217 is opened,while the normally open contact to the same switch is closed. Thecarrier-frequency signal transmitted through line 213, transformer219420, line 224 and transformer 225426 to amplifier 227 and mixer 228,is frequency-modulated with a subaudio frequency, and therefore hassidebands located in the frequency spectrum upon each side of thecarrier frequency and differing therefrom by subaudio frequencydifierences. Accordingly, three components of signal (or more, dependingupon the modulation index) at substantially the carrier frequency aretransmitted simultaneously to mixer 228, and each of these threecomponents is heterodyned with the sideband frequencies transmitted tothe mixer through the .lead 211. Consequently, sidebands are added tothe audio-frequency components supplied through audio amplifier 22? toloud-speaker 230. A mathematical analysis ..will show that thesesidebands are the same as those that would be produced if the originalaudio-frequency signal were frequency-modulated with the same subaudiofrequency as that used to modulate the capacitance of capacitor 233.Consequently, the musical tone produced by loud-speaker 230 isfrequency-modulated with the subaudio frequency, thereby producing avibrato like efiect.

A better understanding of the modulation action may be had byconsidering the instantaneous frequencies of the carrier-frequencysignal supplied to lead 218 and thus to --mixer 228. When thefrequency-modulated carrier is heterodyned with the sidebandstransmitted through 211, the difference frequencies cyclically vary infrequency, and consequentl a frequency-modulated audio signal is If inaddition to the frequency modulation of the carrier, there is also someamplitude modulation of the carrierfrequency signal supplied to mixer223, there .will be a corresponding amount of amplitude modulation ofthe audiosignal supplied to the loud-speaker. In general this is outundesirable, since vibratos produced by conventional musical instrumentscontain a certain amount of tremolo or amplitude modulation. However,the amplitude modulation of the carrier can be suppressed if desired, byproviding in amplifier 227 conventional automatic-volume control oramplitude-limiting means for degenerating or clipping the amplitudemodulation of the carrier.

Now assume that switch 217 is in the position shown in the drawing,while switch 234 is closed. Two carrierfrcquency signals are nowsupplied to mixer 228 simultaneously, one of which is unmodulated whilethe other is frequency modulated as hereinhefore explained. Consequently, the audio-frequency signal supplied to the loudspeaker hasunmodulated components and frequency modulated components simultaneouslypresent, and a thick musical tone is produced that gives a choral effectsimilar to that of a plurality of similar musical instruments playingthe same notes simultaneously.

Another phase or frequency modulator for the carrierfrequency signalcomprises resistors 245 and 246, inductor 247, variable capacitor 243,and a normally open switch 249, connected as shown. The capacitance ofcapacitor 248 is modulated with a subaudio frequency, different from themodulation frequency of capacitor 233, by rotating the movable plates ofcapacitor 248 at a different speed. The movable plates of capacitor 248may be rotated by a drum 248 having a diameter different from that ofdrum 237 that is also in frictional engagement with the driving belt238.

When switches 234 and 249 are both closed, the carrier frequency isphase or frequency-modulated with two different subaudio frequenciessimultaneously, thereby providing an additional number ofcarrier-frequency sidebands each of which produces a beat or differencefrequency when hereterodyned with the sidebands transmitted through lead211. Consequently, when both of the switches 234 and 249 are closed, themusical tone produced by loud-speaker 230 is thicker than when only oneof these switches is closed, and the choral etfect produced represents alarger number of similar musical instruments playing the same notessimultaneously.

Still another phase or frequency modulator for the carrier frequencycomprises a variable capacitor 250 and a normally open switch 251. Thecapacitance of capacitor 25% is varied at still another subaudiofrequency by rotation of the movable plates of capacitor 250 by a drum252 driven by belt 238. When all three of the switches 234, 249, and 251are closed, the carrier-frequency signal is phase or frequency modulatedwith three different subaudio frequencies, thereby producing acorrespondingly larger number of sidebands that result in a stillthicker musical tone.

Still another phase or frequency modulator comprises a variablephase-shifting circuit including a resistor 253. and inductor 254, and avariable capacitor 255 connected in series between lead 222 and theground lead 215. A normally open switch 256 and a resistor 257 areconnected in series between lead 224 and the circuit junction 258 ofresistor 253 and inductor 254. The capacitance of variable capacitor 255is modulated with a subaudio frequency by rotation of the movable platesof the capacitor through a drum 259 driven by belt 233. As hereinbeforeexplained, when switches 234, 249, and 251 are all closed, the carrierfrequency is modulated with three different subaudio frequenciessimultaneously, and lead 218 carries a signal containing the originalcarrier frequency and a number of sidebands. Each of these frequencycomponents is transmitted by the transformer 219- 226 to the leads 221and 222. Consequently, each of these frequency components is itselffrequency-modulated by the phase-shifting network including variablecapacitor 255 to multiply the number of sidehands present. Accordingly,when switch 256 is closed, the number of carrienfrequency sidebandstransmitted to mixer 22% is multiplied to a large number, and theaudio-frequency signal transmitted to loudspeaker 239 has acorrespondingly large number of sidebands. Further additions to thenumber of sidebands can be provided by closing a switch 260 associatedwith still another phase-shifting circuit including a variable capacitor261, and by closing a switch 262 associated with a phase-shiftingcircuit including a variable capacitor 263. The movable plates ofcapacitors 261 and 263 are rotated at different speeds by drums 264 and265 that are in frictional engagement with driving belt 233. The sixdrums driven by belt 238 all have different diameters, so that the sixcapacitoirs 233, 243, 25% 255, 261, and 263 have their capacitancesvaried cyclically at a different subaudio frequency. By closingsuccessively increasing numbers of the switches 234, 249, 251, 256, 260,and 262, an increasingly complex spectrum of sidebands can be providedfor each frequency component of the audiofrequency signal provided bymusical instrument 26-4, thereby producing increasingly thick musicaltones that represent increasingly greater numbers of musical instrumentsplaying simultaneously.

In the description so far it has been assumed that frequency modulationand phase modulation are the same. According to modulation theory thisis true, except that in frequency modulation the modulation index is afunction of the modulating frequency, while in phase modulation it isnot. If all six of the modulators in the FIG. 11 apparatus are madeidentical except for the speed at which the moving plates of thecapacitors are rotated, phase modulation of the carrier will result andthe audiofrequency signal will likewise be phase modulated. In this casethe frequency deviation will be proportional to the modulatingfrequency. in other words, if phase modulation of a 2 cycle per secondmodulating frequency produces a frequency deviation of 10 cycles persecond, then a 6 cycle per second modulating frequency of the sameamplitude will produce a frequency deviation of 30 cycles per second.

For example, assume that the movable plates of capacitor 248 are rotatedthree times as fast as the movable plates of capacitor 233 to providemodulating frequencies of 6 cycles per second and 2 cycles per second,respectively. If these two modulators are made identical except fortheir modulating frequencies, their modulation indices will be identicaland the carrier-frequency signal supplied to lead 218 will be phasemodulated with two subaudio frequencies when both of the switches 234and 249 are closed, and the frequency spectrum occupied by sidebandsproduced by the 6 cycle per second modulation will be three times aswide as that occupied by the sidebands produced by the 2 cycle persecond modulation.

To produce frequency modulation rather than phase modulation, is onlynecessary to make the modulation indices of the different modulatorsinversely proportional to the modulating frequencies. This can easily bedone by an appropriate design of the modulators. For example, if theresistance of resistor 245 is made three times as large as that ofresistor 231, the modulation indices of the two modulators will besubstantially inversely proportional to the modulation frequencies, andthe frequency spectrum occupied by sidebands produced by the 6 cycle persecond modulation will be substantially the same Width as the frequencyspectrum occupied by sidebands produced by the 2 cycle per secondmodulation. In this case the carrier frequency is frequencymodulatedwith the two subaudio frequencies when both of the switches 2-34 and 249are closed. Generally, frequency modulation is preferred to phasemodulation, but since some mixture of phase as well as amplitudemodulation with the frequency modulation is not generally objectionablein a musical tone, the modulation indices need not be as carefullyadjusted as would be required in a high-quality frequency-modulationcommunications system.

The apparatus shown in FIG. 11 can be modified in various ways withoutdeparting from the broader inven tive principles involved. For example,multiple carriers can be produced from a single carrier by amplitudemodulation of the carrier in place of frequency or phase modulation.Instead of supplying an unmodulated carrier to the modulator 2G8 andsupplying a modulated carrier to mixer 22 3, the modulated carrier maybe supplied to modulator While an unmodulated carrier is supplied tomixer 228. Many different types of oscillators, modu- 23 lators,filters, amplifiers, and mixers may be employed, there being manysuitable types that are Well known in the communications art.

The several forms of tone modifiers here described are compatible andmay, if desired, be used together. For example, the pitch modulatorillustrated in FIGS. 1 through 6 may be connected in tandem with theamplitude modulator illustrated in FIG. 7 and the timbre modulatorillustrated in PEG. 9. 'With such arrangement a great variety of musicaleffects can be created at will by the musician. Other tandem andparallel combinations of various modulators can be employed, includingcombinations of the carrier types with the direct modulation types.

Instead of using discrete subaudio modulating frequencies, spectra ofsubaudio modulating frequencies may be employed. For example, if arandom noise signal is transmitted through a low-pass filter having acut-oil frequency of approximately seven cycles per second, an infinitenumber of subaudio frequencies all within the range of zero to sevencycles per second is obtained, and the original audio-frequency signalmaybe modulated, either directly or indirectly, by this spectrum ofsubaudio frequencies to produce a continuous narrow spectrum ofsidebands about each audio-frequency component. Such modulation bysubaudio noise is equivalent to modulation by an infinite number ofdiscrete subaudio frequencies.

It should be understood that this invention in its broader aspects isnot limited to specific embodiments herein illustrated and described,and that the following claims are intended to cover all changes andmodifications that do not depart from the true spirit and scope of theinvention.

What is claimed is: V

1. Apparatus for providing an audible signal comprising,

sources of at least three periodic signals separated by at least threedifferent subaudio frequency differences, at least one of said signalshaving a frequency in the audible range,

and means for modulating said one periodic signal with n I the remainingones of said signals to provide an output signal having adjacentspectral components separated by subaudio frequencies in a bandembracing said audible range frequency and separated by at least saidthree different subaudio frequency differences to provide a choral tonesignal characterized by an aurally untrackable modulation pattern.

2. Apparatus in accordance with claim 1 and further comprising,

transducing means for converting an electrical signal to an acousticalsignal,

and means for coupling said modulating means output signal to saidtransducing means to provide an audible choral tone. 7 3. Apparatus forproviding an audible signal comprising,

a source of musical tone signal, sources of periodic modulating signalsestablishing at least three different subaudio modulating frequencies,modulating means, means for coupling said musical tone signal source andsaid sources of said modulating signals to said modulating means, saidmodulating means being responsive to said modulating signals formodulating said musical tone signal to provide a choral tone audiooutput signal having a spectrum embracing the frequency of said musicaltone signal with sidebands separated by said subaudio frequencies, saidoutput signal having a complex modulation pattern with a period which islong compared to that of a musical tone.

4. Apparatus in accordance with claim 3 wherein said modulating meanscomprises,

a magnetic recording medium, a

means for recording a signal related to said musical tone signal uponsaid recording medium,

means responsive to said recorded signal on said magnetic recordingmedium for providing said output signal,

and means for establishing relative cyclical movement at at least saidthree different subaudio frequencies between said means recording saidsignal on said medium and said means responsive to said recorded signal.

5. Apparatus in accordance with claim 4 wherein said means for recordingand said means for providing an output signal comprise a plurality ofelectromagnetic transducing heads,

means for establishing relative movement between said heads and saidrecording medium to exchange signals therebetween,

and means for establishing relative cyclical motion between at least oneof said recording heads and at least one of said reproducing heads at atleast said three different subaudio frequencies.

6. Apparatus in accordance with claim 3 and further comprising,

ransducing means responsive to a final output signal for providing acorresponding acoustical signal simulating a choral tone,

and a plurality of said modulating means jointly energized by saidmusical tone signal and each providing a said output signal,

means for combining the last-mentioned output signals to provide saidfinal output signal,

and means for coupling said final output signal to said transducin gmeans.

7. Apparatus in accordance with claim 6 and further comprising,

means for selectively controlling the number of said dii erent suoaudiomodulating frequencies.

8. Apparatus in accordance with claim 3 wherein said modulating meanscomprises,

at least three parallel variable attenuators energized by said musicaltone signal,

' and means for cyclically varying the attenuation imparted to saidmusical tone signal by each attenuating means at a respective one ofsaid subaudio frequencies.

9. Apparatus in accordance with claim 8 wherein each of said attenuatingmeans comprises a variable reactance.

1G. Apparatus in accordance with claim 8 wherein each of saidattenuators comprises a variable inductance.

11. Apparatus in accordance with claim 3 wherein said sources ofmodulating signals comprises sources of at least three superaudiosignals separated from one another by said subaudio frequencies.

12. Apparatus in accordance with claim ll wherein said modulating meanscomprises a first modulator energized by said musical tone signal and atleast three of said superaudio frequency signals to provide a signalhaving spectral components in the superaudio frequency range,

an a mixer energized by the latter signal and one of said superaudiofrequency signals to provide said output signal.

13. Apparatus in accordance with claim l2 and further comprising,

a source of at least two superaudio f equency signals separated by asubaudio dif erence frequency,

a second modulator energized by said last-mentioned output signal and atleast the latter two superaudio frequency signals to provide anintermediate output signal having spectral components in the superaudiofrequ ncy range separated by all said subaudio'frequency differences,

1. APPARATUS FOR PROVIDING AN AUDIBLE SIGNAL COMPRISING, SOURCES OF ATLEAST THREE PERIODIC SIGNALS SEPARATED BY AT LEAST THREE DIFFERENTSUBAUDIO FREQUENCY DIFFERENCES, AT LEAST ONE OF SAID SIGNALS HAVING AFREQUENCY IN THE AUDIBLE RANGE, AND MEANS FOR MODULATING SAID ONEPERIODIC SIGNAL WITH THE REMAINING ONES OF SAID SIGNALS TO PROVIDE ANOUTPUT SIGNAL HAVING ADJACENT SPECTRAL COMPONENTS SEPARATED BY SUBAUDIOFREQUENCIES IN A BAND EMBRACING SAID AUDIBLE RANGE FREQUENCY ANDSEPARATED BY A LEAST SAID THREE DIFFERENT SUBAUDIO FREQUENCY DIFFERENCESTO PROVIDE A CHORAL TONE SIGNAL CHARACTERIZED BY AN AURALLY UNTRACKABLEMODULATION PATTERN.